SQL Correlated Subquery

Summary: in this tutorial, you will learn about the SQL correlated subquery which is a subquery that uses values from the outer query.

Introduction to SQL correlated subquery

Let’s start with an example.

See the following employees the table in the sample database:

The following query finds employees whose salary is greater than the average salary of all employees:

SELECT 
    employee_id, 
    first_name, 
    last_name, 
    salary
FROM
    employees
WHERE
    salary > (SELECT 
            AVG(salary)
        FROM
            employees);

In this example, the subquery is used in the WHERE clause. There are some points that you can see from this query:

First, you can execute the subquery that returns the average salary of all employees independently.

SELECT 
    AVG(salary)
FROM
    employees;

Second, the database system needs to evaluate the subquery only once.

Third, the outer query makes use of the result returned from the subquery. The outer query depends on the subquery for its value. However, the subquery does not depend on the outer query. Sometimes, we call this subquery is a plain subquery.

Unlike a plain subquery, a correlated subquery is a subquery that uses the values from the outer query. Also, a correlated subquery may be evaluated once for each row selected by the outer query. Because of this, a query that uses a correlated subquery may be slow.

A correlated subquery is also known as a repeating subquery or a synchronized subquery.

SQL correlated subquery examples

Let’s see a few more examples of the correlated subqueries to understand them better.

SQL correlated subquery in the WHERE clause example

The following query finds all employees whose salary is higher than the average salary of the employees in their departments:

SELECT 
    employee_id, 
    first_name, 
    last_name, 
    salary, 
    department_id
FROM
    employees e
WHERE
    salary > (SELECT 
            AVG(salary)
        FROM
            employees
        WHERE
            department_id = e.department_id)
ORDER BY 
    department_id , 
    first_name , 
    last_name;

Here is the output:

In this example, the outer query is:

SELECT 
    employee_id, 
    first_name, 
    last_name, 
    salary, 
    department_id
FROM
    employees e
WHERE
    salary >
...

and the correlated subquery is:

SELECT
    AVG( list_price )
FROM
    products
WHERE
    category_id = p.category_id

For each employee, the database system has to execute the correlated subquery once to calculate the average salary of the employees in the department of current employee.

SQL correlated subquery in the SELECT clause example

The following query returns the employees and the average salary of all employees in their departments:

SELECT 
    employee_id,
    first_name,
    last_name,
    department_name,
    salary,
    (SELECT 
            ROUND(AVG(salary),0)
        FROM
            employees
        WHERE
            department_id = e.department_id) avg_salary_in_department
FROM
    employees e
        INNER JOIN
    departments d ON d.department_id = e.department_id
ORDER BY 
    department_name, 
    first_name, 
    last_name;

The output is:

For each employee, the database system has to execute the correlated subquery once to calculate the average salary by the employee’s department.

SQL correlated subquery with EXISTS operator example

We often use a correlated subquery with the EXISTS operator. For example, the following query returns all employees who have no dependents:

SELECT 
    employee_id, 
    first_name, 
    last_name
FROM
    employees e
WHERE
    NOT EXISTS( SELECT 
            *
        FROM
            dependents d
        WHERE
            d.employee_id = e.employee_id)
ORDER BY first_name , 
         last_name;

The following picture shows the output:

In this tutorial, you have learned about the SQL correlated subquery and how to apply it to form a complex query.